A multitude of wireless communication systems support wide ranging needs. Many wireless communication systems supporting the same needs are incompatible. For example, there are several wireless telephone communication standards, each configured to support consumer wireless telephone communications. There are, for example, Code Division Multiple Access (CDMA) telephone systems, Time Division Multiple Access (TDMA) telephone systems, and analog telephone communication systems. Each type of telephone system may have numerous different standards for that particular type of system. However, many of the communication standards do not support interoperability with other communication standards.
There is a desire to implement a unified user device that has the ability to communicate over multiple communication systems, each supporting a distinct communication standard. Simultaneously, there is a desire to decrease the physical size and cost of the user devices, while increasing its complexity.
One manner of reducing the cost of a user device is to minimize the component count, and in particular, minimizing the number of higher priced components used in the assembly. One manner of reducing parts in a transmitter or receiver is to implement a direct conversion architecture rather than a superheterodyne architecture. A direct conversion architecture has a lower component usage when compared to the superheterodyne architecture.
However, there may be performance disadvantages associated with the direct conversion architecture, particularly when implemented in a transmitter. The disadvantages of the typical direct conversion transmitter are most evident in full duplex communication systems, where the transmitter and receiver operate simultaneously. An example of such a full duplex system is a CDMA wireless communication system. In some full duplex communication systems, the transceivers must have greater than 75 dB of dynamic range. In a CDMA communication system, the dynamic range requirement is a result of the near-far problem apparent in cellular systems.
The typical direct conversion transmitter architecture cannot meet the dynamic range requirement without requiring an external band pass filter to reduce the noise during full duplex operation. A contributor that prevents the implementation of reduced noise direct conversion architectures with >75 dB of dynamic range is carrier feedthrough.
An issue with the typical direct conversion architecture is that multiple stages of RF variable gain amplifiers are used to implement the dynamic range of the transmitter. These stages dramatically increase the noise, typically increasing the noise by greater than 20 dB over the thermal noise floor, during duplex operation and thus RF band pass filters are required. Typically, multiple RF gain stages and filters are used due to the high level of carrier feedthrough at the output.
Superheterodyne architectures present their own disadvantages. The primary deficiency of the superheterodyne architecture is high circuitry/component count. Specifically, the implementation of this architecture requires an extra oscillator, such as a Voltage Controlled Oscillator (VCO) and Intermediate Frequency (IF) band pass filter. Additionally, the RF Variable Gain Amplifier (VGA) needed to implement the dynamic range is noisy, thus requiring an RF bandpass filter for duplex operation. Because of these negative aspects of this architecture, state of the art transmitters typically do not utilize superheterodyne architectures.
Thus it is desirable to implement efficient low noise architectures that can support a host of difficult requirements, including low noise and low carrier feedthrough.